elementary processes of soil–water interaction and thresholds in soil surface dynamics: a review

PROCESSES OF SOIL–WATER INTERACTION 1077

Copyright © 2004 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms 29, 1077–1091 (2004)

Earth Surface Processes and Landforms

Earth Surf. Process. Landforms 29, 1077–1091 (2004)Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/esp.1103

ELEMENTARY PROCESSES OF SOIL–WATER INTERACTION ANDTHRESHOLDS IN SOIL SURFACE DYNAMICS: A REVIEW

RICHARD S. B. GREENE1* AND PETER B. HAIRSINE2

1 CRC for Landscape Environments and Mineral Exploration, School of Resources, Environment and Society, Australian National

University, Canberra, ACT 0200, Australia2 CRC for Catchment Hydrology, CSIRO Land and Water, GPO Box 1666, Canberra, ACT 2601, Australia

Received 28 January 2002; Revised 19 February 2004; Accepted 4 March 2004

ABSTRACT

Elementary processes of soil–water interaction and the thresholds to these processes are important to understand as theycontrol a range of phenomena that occur at the soil surface. In particular processes involved with wetting by rainfall thatlead to particle breakdown are critical. This breakdown causes soil detachment and crust formation, which are both keyelements in erosion.

This paper reviews the range of approaches that have been taken in describing the processes associated with the wettingof a soil surface by rainfall. It assembles the studies that emphasize soil physics, soil chemistry, and erosion mechanics ina framework to enable a balanced consideration of important processes and management strategies to control erosion for aparticular situation. In particular it discusses the factors associated with the two basic processes of soil structural breakdown,i.e. slaking and dispersion, and how these processes are critical in particle detachment, transport and surface crust formation.Besides the balance between the exchangeable cation composition and electrolyte concentration (measured as the sodiumadsorption ratio (SAR) and total cation concentration (TCC) respectively) of the soil, the importance of energy input andsoil organic matter content in controlling clay dispersion is emphasized.

Based on the balance between these factors, the soil can be in one of three different regions, i.e. a dispersed region,a flocculated region and one where the resilience of the soil is variable. The implications of each of these regions to soilerosion management are briefly outlined, as are the critical roles that soil cover levels and organic matter have in controllingerosion. Finally, the relationship between various laboratory measures of aggregate stability, and corresponding field erosioncharacteristics, is discussed. Copyright © 2004 John Wiley & Sons, Ltd.

KEY WORDS: swelling; slaking; clay dispersion; cover; aggregate stability; surface crust; sealing

INTRODUCTION

As a major form of land degradation, soil erosion by rainfall and runoff has attracted the attention of researchersfrom a range of disciplines. Each of these disciplines has contributed to the body of knowledge concerning theresponse of soil to wetting by raindrops and subsequent detachment, infiltration and runoff processes. Soilchemists, soil physicists and a large number of other specialists investigating the mechanics of raindrop-drivenerosion, have all analysed aspects of these phenomena.

In Table I we list some of the different attributes that are considered important from the viewpoints of soilchemistry, soil physics and erosion mechanics.

While the three viewpoints emphasize different attributes contributing to the erosion process, there are regionsof overlap. However, to most specialists and the researchers attempting to use this body of research, theknowledge is somewhat disconnected and suffers from a complex, poorly structured discussion across the manyviewpoints. Yet the need to understand the soil surface dynamics of erosion is greater than ever, as this formof land degradation affects large parts of the world’s land surface. For example Figure 1 demonstrates a badlygullied area in the Upper Murrumbidgee River Catchment, southeast of Canberra, ACT, Australia. The soilprofile has highly dispersive sodic subsoils, which results in sidewall fluting and the development of tunnelling

* Correspondence to: R. S. B. Greene, CRC for Landscape Environments and Mineral Exploration, School of Resources, Environment andSociety, Australian National University, Canberra, ACT 0200, Australia. E-mail: [email protected]

1078 R. S. B. GREENE AND P. B. HAIRSINE


Table I. Different viewpoints to studying erosion

Viewpoints Attributes contributing to soil erosion by water

Soil chemistry Clay chemistry effects on swelling, dispersion/flocculationAggregate stability as affected by organic matterSodicity and salinitySlaking and rate of wetting effects

Soil physics Hydraulic conductivity of surface layersCrust formation/sealingSlakingInfiltration/runoffWater balance

Erosion mechanics Infiltration/runoffDetachment/transportSediment concentrationSoil erodibility

Figure 1. Badly gullied area formed on dispersive sodic subsoils in the Upper Murrumbidgee River Catchment, south east of Canberra,ACT, Australia (photo courtesy of J. James)

(J. James, pers.comm., 2004). Such forms of land degradation are common in many areas of Australia affectedby dispersive subsoils.

The aim of this paper is to provide a common framework for considering the soil surface dynamics of erosionunder rainfall across these three viewpoints, and understanding how the attributes in each of three viewpointsfit together. The key to this integration is to investigate the interaction of water with soil particles. This isbecause in erosion studies, we are primarily concerned with the consequences of rainfall on a land surface, andthe factors associated with water interacting with soil at a range of scales. However, as the proposed frameworkmainly concentrates on the processes involved in the breakdown of soil aggregates at the surface, it has particu-lar relevance to interrill erosion.

The review begins by considering the consequences of rainfall on the soil surface, and the physical andchemical changes that occur due to the interaction of water with the soil particles. A framework is presentedfor the different stability states of a soil in relation to how the physico-chemical factors, energy inputs andorganic matter content influence clay dispersion. The dispersive potential of the soil is critical to understand, as



it is the major factor controlling surface crusting and sealing, particle detachment and hence erosion potential.The management considerations for the different stability states are then discussed, as is the role of differentcover levels and types and organic matter in preventing particle detachment due to dispersion. The final sectiondeals with the relationship between various laboratory tests of soil stability and their corresponding fieldbehaviour, particularly in relation to the aggregate breakdown processes of slaking and dispersion and theirconsequences on particle detachment and sediment concentration.

CONSEQUENCES OF RAINFALL ON A SOIL SURFACE

When rain falls on a soil surface the soil structure may alter and sediment may be released. This alteration insoil structure and release of particles results from a series of interrelated physical and chemical changes. Thecombined physical and chemical changes to the soil surface are also strongly coupled to the generation ofoverland flow and the detachment of sediment from the soil surface (Romkens et al., 1997). For example, surfacesealing as a result of physical changes leads to more rapid saturation of the surface layer and an earlier onsetof ponding, greater runoff rate, finer sediment supplied, reduced surface roughness and a change in surfaceresistance to detachment. While most studies suggest that seal formation accentuates both the rate and extentof erosion, there are several studies that suggest there are also retarding mechanisms, such as increasing the soilsurface’s resistance to detachment (Levy et al., 1994). To fully assess the consequences of rain on a soil surface,it is necessary to consider the range of physical and chemical changes that can occur.

Physical changes

The landmark study of Moss (1991a,b,c) and Moss and Watson (1991) drew together much of the knowledgeconcerning the physical changes of the soil surface due to rainfall. The study was unique at that time as itdrew on real time observations, sequential sampling of micro-morphology using thin sections and small-scale,laboratory, rainfall erosion simulation techniques. While the studies that are reported in these papers are anoriginal experimental study, the introduction and discussion form a framework that enables the consideration ofsimilar studies up until that time. Moss (1991a) constructed a framework describing a series of stages throughwhich the soil surface passed from its initial condition to the sealed surface that generated overland flow withthe potential to accelerate soil erosion. Each of these stages is now considered in turn.

In stage one the surface is first exposed to rainfall in an unsaturated, often dry state. The first impacts ofrainfall result in the water entering the soil surface pores at considerable speed, termed hydraulic jetting. Air inthe affected pores is compressed or displaced so that some changes to the structure of the surface soil are made.Jets of air carrying some small amounts of dust were observed in this stage. In the study of Moss (1991a) thisstage lasted approximately 30 seconds for simulated rain with near-terminal velocity, drop sizes of 5·1 mm anda rainfall intensity of 40 mm h−1.

In stage two many of the surface pores become saturated and raindrop impact is accompanied by intensedisruption of the soil surface structure. Aggregates are broken rapidly and soil pores are reduced in volume andcontinuity. The change in soil structure provides a positive feedback towards the onset of stage three. Craterformation and intense air splash also often accompany stage two.

In stage three the soil surface structure has degraded to the extent that pores that can conduct water rapidlyfrom the surface are no longer present and ponding commences. The surface then becomes affected by thelocally intense shearing forces associated with the horizontal dissipation of raindrop impacts. As ponding de-velops the layer of surface water lessens the interaction between the incoming drops and the degraded surface(Kinnell, 1990). This observation is supported by the study of Nearing et al. (1987).

The consequences of the disruption of the soil surface structure for the hydraulic properties of the soil havebeen extensively investigated (see review by Bristow et al., 1995). Investigations have focused on the layeredsystem, which forms as a result of the development of a crust or seal above structurally unaltered soil. Forexample, Moore (1981) described the development of a seal of finite depth by changing the hydraulic conduc-tivity of the surface layer. More sophisticated models of the hydraulic behaviour have also been developed. Forinstance Mualem et al. (1993) developed and evaluated a model of a soil profile with a dynamic crust thatchanged the water content function, the water retention curve and the hydraulic conductivity function. These



developments quantitatively describe the qualitative observations of Moss (1991a) so that they may be describedin a deterministic water balance model.

Moss (1991a) also observed that the surface of the soil came to have a partial shield of silt-sized material.This observation is consistent with the theory of Hairsine et al. (1999) who invoke a deposited layer of detachedmaterial as a storage, which is significant in the observed size selectivity sediment transport in the early phasesof a rainfall-driven erosion event. This well-sorted layer was observed by Heilig et al. (2001) in a study focusedon the size selectivity of rainfall-driven erosion. Interestingly Moss (1991a) dismisses previous seal formationmechanisms proposed by McIntyre (1958a,b) as an experimental artifact. It is suggested that the sub-millimetreobserved skin seal was a function of the closed nature of the horizontal soil surface used in this study and formedat the completion of the applied rainfall when resuspension ceased.

It is important to note that the studies described above deal almost exclusively with structural crusts that areformed in situ. Depositional crusts are associated with sediment detached from the soil surface and depositedin a size-sorted manner (West et al., 1992). These crusts occur as a consequence of sediment transport and tendto be spatially patchy (Bresson, 1995). Their effect on soil hydraulic properties is similar to structural crusts(Shainberg and Singer, 1986). In addition, significant research on the various stages of aggregate breakdown,surface seal development and crusting on a range of different soils has been carried out by Farres (1987) andBresson and Valentin (2001).

Having introduced the physical changes resulting from water interacting with the soil surface, it is nownecessary to consider the chemical interaction of water with soil surfaces and the consequences of these inter-actions on soil properties such as swelling, slaking and dispersion. Though we present the physical and chemicalchanges sequentially, they occur simultaneously and are strongly coupled. We start by considering the chemicalinteraction of water and clays.

Chemical changes

The following section discusses how the interaction of water with soil surface particles forms the basis forthe phenomena of swelling, slaking and clay dispersion. When clay particles in the soil interact with water, theyinitially undergo swelling (Quirk, 1994). In some cases this leads ultimately to the disruption of soil aggregatesinto finer particles, which become detached and can therefore erode. This disruption can take the form of eitherslaking and/or dispersion. Slaking results when air-dry aggregates are rapidly wet up, as occurs in stages oneand two described above, compressing air inside the aggregates, which causes failure (Torri and Borselli, 2000).Dispersion is a result of continued interlayer swelling where the clay particles become separated into distinctindividual entities. Rengasamy and Sumner (1997) have given an excellent account of how clay particlesundergo a combination of swelling, slaking and dispersion in water.

A range of detailed soil chemistry studies has been carried out on the interaction of water with clay particlesand the consequences for soil stability. However, before we discuss these interactions and consequences of thesebreakdown processes on detachment and hence on the propensity of a soil particle to be transported, it isnecessary to consider the organization of particles in a soil, particularly the clay fraction.

Clay domains and interparticle forces. In clay soil systems, the plate-shaped clay particles are assembled intoparallel or near-parallel alignment to form larger units referred to as ‘clay domains’ (Quirk, 1994; Quirk andMurray, 1991). In pure calcium (Ca) systems there is a near-parallel alignment of clay particles, strong overlapand the formation of clay domains with the individual clay particles existing in a stable, deep, primary potentialenergy minimum. The clay particles remain in these potential minima, swelling is limited to 19 Å, and isindependent of electrolyte concentration. However, in sodium (Na) systems or mixed Ca/Na systems, swellingincreases with a decrease in the electrolyte concentration, the clay particles ultimately dispersing into separateentities (Rengasamy and Sumner, 1997). From electrical double-layer computations, it can be shown that thedouble layer thickness (1/κ), which controls swelling, is proportional to 1/√c, where c is the molar electrolyteconcentration (Van Olphen, 1977). Table II shows that the swelling of Ca montmorillonite (as measured by X-ray spacing) does not change with a decrease in electrolyte concentration, whereas Na montmorillonite under-goes extensive crystalline swelling due to the development of diffuse double layers with a decrease in electrolyteconcentration.



Table II. X-ray spacings for Ca and Na montmorilloniteclays in CaCl2 and NaCl electrolyte solutions respectively

(adapted from Quirk, 1968)

Solution X-ray spacing (Å)

0·1 M CaCl2 19·0<10–5 M CaCl2 19·01·0 M NaCl 18·70·5 M NaCl 18·90·3 M NaCl 43·0

Figure 2. Effect of sodium adsorption ratio (SAR) and electrolyte concentration on soil permeability (adapted from Quirk and Schofield, 1955,by permission of Blackwell Publishing)

Threshold concentration and dispersive soils. The effects of this increase in swelling with decreasing electrolyteconcentration for mixed Ca/ Na systems and its ultimate effect on soil physical properties (such as permeability)was investigated by Quirk and Schofield (1955). Using a permeability test, they measured the minimum levelof electrolyte required to maintain an illitic loam soil in a permeable condition for a given degree of sodiumsaturation of the soil colloids, called the ‘threshold concentration’ (Figure 2). The soil was brought to equilibriumwith relatively concentrated mixed NaCl/CaCl2 solutions, having different sodium adsorption ratios (SAR),where the SAR is the ratio (Na+)/(Ca2+)0·5. Once exchange equilibrium was obtained, then the solution wasdiluted, but the SAR kept constant. The threshold concentration CT was taken as the concentration when therewas a 15 per cent decrease in permeability, and was related to the SAR as shown in Equation 1:

CT = 0·56 SAR + 0·6 (1)

Below this concentration the permeability continued to decrease due to dispersed clay. The electrolyte con-centration CD at which dispersed clay was visible in the percolating solution was approximately one-quarter ofthe threshold concentration CT and is given by Equation 2:

CD = 0·14 SAR + 0·20 (2)

Rengasamy et al. (1984) developed this concept of threshold concentration using clay soil–water suspensionsand obtaining the relationship between the electrolyte concentration expressed as the total cation concentration(TCC) and the SAR of the suspensions that distinguished a dispersed state from a flocculated state (Figure 3).



Figure 3. Effect of sodium adsorption ratio (SAR) and electrolyte concentration (expressed as total cation concentration, TCC) on claydispersion (adapted from Rengasamy et al., 1984, by permission of CSIRO Publishing)

They also suggested that the TCC (measured as milli-equivalents l−1 (m.e. l−1)) may be reliably estimated fromthe electrical conductivity (EC25) (measured as dS m−1) of the 1:5 soil–water suspension using the relationshipin Equation 3:

TCC = 9·62 EC25 + 0·14 (R2 = 0·97) (3)

The threshold concentration was measured under two energy conditions, corresponding to low energy, spontaneousdispersion or high energy, mechanical dispersion. The low-energy condition was achieved in the laboratory bygently pipetting water down the sides of a bottle containing soil, and then gently stirring the water to suspendthe clay. In contrast the high-energy condition was achieved by shaking end-over-end a 1:5 soil–water suspensionfor one hour, and then measuring the amount of dispersed clay. Rengasamy et al. (1984) proposed that the twoenergy conditions represent a soil surface protected from raindrop impact by plant cover, mulches etc., and oneexposed directly to raindrop impact, respectively. The equations for the two energy conditions of spontaneousand mechanical dispersion are given by Equations 4 and 5 respectively. It is interesting to note that the conditionsfor spontaneous dispersion, Equation 4, are similar to those found by Quirk and Schofield (1955) for dispersedclay to appear (Equation 2):

TCC = 0·16 SAR + 0·14 spontaneous dispersion (4)

TCC = 1·21 SAR + 3·3 mechanical dispersion (5)

Ford et al. (1985) used the high-energy mechanical dispersion technique to test the effect of soil organic mattercontent (as indicated by different types of land use) on the TCC/SAR relationship. Using soils of decreasingorganic matter content, i.e. pasture to wheat-fallow to continuously cropped, they showed that with lowerorganic matter content, a higher electrolyte concentration was required to maintain flocculation for a given SARlevel (Figure 4). Typical values for soil organic carbon levels in a pasture, wheat-fallow and continuouslycropped soil are 3–4 per cent, 1·5–2 per cent and <1·5 per cent respectively (Rovira, 1993). Thus the organicmatter appears to be stabilizing the soil aggregates against disruption from dispersion.

Figure 5 is a schematic diagram (developed by Sumner, 1995) demonstrating the role of mechanical energyand organic matter content in the flocculation/dispersion process, and therefore how the position of the thresholdconcentration line can change according to these two inputs. Thus an increase in mechanical energy inputs ora decrease in organic matter would cause the line to move towards the electrolyte concentration (EC) axis. Thishas the effect of increasing the area of conditions under which dispersed conditions can occur. Figure 5 also



Figure 4. Effect of organic matter on clay dispersion (adapted from Ford et al., 1985)

Figure 5. Role of organic matter and energy input in clay dispersion (adapted from Sumner, 1995, by permission of CSIRO Publishing)

illustrates how there are three different regions each corresponding to a different stability state for a soil, i.e.dispersed, flocculated and variable resilience. The characteristics of each of these regions are as follows.1. In the dispersed region:

(i) soil chemistry, i.e. the balance between SAR and EC, is critical; and(ii) the management implications are that these soils should be left undisturbed in a native or lightly grazed state,

or if irrigated, an electrolyte (such as gypsum) can be added to the irrigation water so that the soil movesto the variable resilience zone in Figure 5.

2. In the flocculated region:

(i) soils are chemically stable and resist crusting (Rengasamy and Sumner, 1997): and(ii) the management implications are to maintain organic matter levels and/or use normal soil conservation

practices.

3. In the variable resilience region:

(i) soil behaviour varies with physical and mechanical attributes; and



(ii) management options are: (a) in the short term to increase cover levels (and hence decrease the amount oferosion). Increasing cover levels is analogous to less mechanical energy input; and (b) in the long term toimprove organic matter levels in the soil. Both these approaches are well-known methods used in the controlof erosion. Each is discussed briefly.

SHORT-TERM MANAGEMENT EFFECTS OF COVER LEVELS AND TYPES ON EROSION

General effects of cover on particle detachment and sediment formation

Cover of various forms can be used to protect the soil surface from raindrop impact. The amount of detach-ment caused by a drop is related to the kinetic energy of the drop at the moment of impact, and is given byEquation 5 (Torri and Borselli, 2000):

Ds = Ks(E − Eo) (6)

where Ds is soil detachment (kg m−2), E is kinetic energy at impact (kJ m−2) and Eo is the threshold energy toinitiate the detachment process, and Ks is soil detachability (kg kJ−1).

Because cover decreases the kinetic energy of the rain that is released and dissipated to the soil surface, itreduces the amount of detachment, and hence erosion that can occur. Figure 6 shows a typical exponentialresponse of decreasing erosion with an increase in soil cover. In terms of the SAR/EC diagram, the increasein soil cover corresponds to an increase in the range of conditions under which the soil surface is stable todispersion and therefore less likely to crust or seal. Cover may also maintain higher soil surface moisturecontents for a significant proportion of time. This effect is useful in reducing the rate of wetting beneath coverand hence the tendency to slake (Chan and Mullins, 1994). The effect of rate of wetting on slaking has also beenextensively investigated by Quirk and Panabokke (1962).

Effect of cover type on erosion

There have been a considerable number of studies on the influence of various forms of cover on the formationof crusts and seals. This cover can be a canopy cover of vegetation, or contact cover such as a mulch or shadecloth, or cryptogams (mixtures of mosses, lichens, algae, liverworts etc), which form an intimate association

Figure 6. Effect of cover on soil erosion



Figure 7. Effect of cryptogam cover on splash erosion (adapted from Eldridge and Greene, 1994b, reproduced by permission of Elsevier)

Figure 8. A summary of five studies relating the sediment concentration in overland flow to the percentage of cover in contact withthe surface

with the soil surface (Eldridge and Greene, 1994a), or rock fragments (Poesen et al., 1994). Vegetative and contactcovers fulfil a range of roles in mitigating soil surface structure decline and erosion under raindrop impact.Clearly cover absorbs some of the energy of raindrop impact, though drops that fall from vegetation still mayhave a significant eroding effect (Moss and Green, 1987). Greene and Ringrose-Voase (1994), however, showed thatwhen shade cloth was placed over the soil surface, there was a five-fold decrease in the erosion rate comparedwith a bare soil surface. With cryptogams, they both protect the surface from raindrop impact and also physicallybind the soil together. Eldridge and Greene (1994b) showed that as cryptogam cover increases on a red earth soil,there was an exponential decline in total splash erosion (Figure 7). Analysis of the eroded material also showedthat there was an increase in the proportion of coarse material in the sediment with an increase in cryptogam cover.

Figure 8 shows a sample of the published studies concerning the effect of cover on soil erosion. These studieshave been selected as they specifically use cover that is in contact with the soil surface. This type of cover isfrequently termed contact cover and is useful because it eliminates effects such as gravity drops from canopy



vegetation and entrainment by overland flow beneath cover. The data in Figure 8 shows a general trend ofreducing sediment concentration with increasing contact cover. However there is a large spread in the data andsome experimental runs resulted in higher sediment concentration with cover that those conducted in identicalexperimental conditions without cover. The authors note various effects including the shielding of underlyingsoil from rainfall impact, the reduced velocity of overland flow from the increased hydraulic roughness providedby cover, the increased mean depth of water on the surface associated with increasing cover which also lessensraindrop impact on the surface, and the local concentration of overland flow as a result of the diversion ofoverland flow paths by cover elements.

Lattanzi and Meyer (1974) found a linear relationship between the total volume of overland flow and the massper unit area of cover added to the surface. For the same experiment the authors found an exponential declineof total soil loss with mass per unit area of cover added to the surface. These results were consistent across arange of surface slopes for the 0·61 m by 0·61 m plots used. These trends suggest that the effect of cover onsoil erosion at this spatial scale is a combination of moderating changes in soil surface hydraulic properties andmoderating of the detachment and transport of sediment for a given volume of overland flow.

Mannering and Meyer (1963) conducted a study similar to that of Lattanzi and Meyer (1974). However, theplots in the earlier study were 10·7 m long and 3·7 m wide and had a gradient of 5 per cent. It is expected thatthe dimensions of these plots would result in thresholds of overland flow-driven erosion being exceeded. Highlevels of mulch application were associated with no runoff. However, for those plots that did have runoff, thesimilar types of responses in runoff volume and soil loss were found for varying cover levels as reported inLattanzi and Meyer (1974).

It is apparent from comparing the studies shown in Figure 8 that the influence of cover is complex and variesbetween soils and other environmental parameters. It can be concluded that decline in soil loss with increasingcontact cover is universal but the reasons for, and shape of, this trend vary. Also, the overall effect of differenttypes of cover on crust formation will be influenced by the relative impacts of processes driven by rate of wettingand those driven by raindrop impact. The relativity of these effects varies considerably across different soils anddifferent antecedent soil–water contents (Loch, 1994; Geeves et al., 1995; Fox and Le Bissonnais, 1998) andwarrants further investigation. For example, Poesen et al. (1999) also demonstrated that the effect of rockfragment cover on sediment concentration was highly dependent on antecedent soil moisture content. For drysoils, the decline in sediment concentration with increasing rock fragment cover was less compared to wet soils,probably because of slaking effects.

LONG-TERM MANAGEMENT EFFECTS OF INCREASING ORGANIC MATTER TO INCREASEAGGREGATE STABILITY AND HENCE DECREASE DETACHMENT

The role of organic matter in stabilizing aggregates against breakdown in water, particularly from slaking, is awell-known phenomenon (Chan and Mullins, 1994). For example, Quirk and Panabokke (1962) demonstratedthat if aggregates from a cropped soil were rapidly wet up from air-dry, they underwent failure (called incipientfailure), which caused them to slake when subsequently immersed in water, whereas aggregates from a virginsoil were stable and remained intact. The deleterious effects of rapid wetting of air-dry aggregates from a croppedfield (compared to wetting of aggregates from a virgin soil) are seen in Figure 9. However, if the aggregatesfrom the cropped soil are slowly wet up, the failure is eliminated, and the aggregates do not undergo slakingwhen immersed in water. This illustrates the importance of increasing the soil organic matter content as a long-term strategy to lower the tendency for soil particles to become detached (as a consequence of both slaking and/or dispersion processes) and hence undergo erosion. It also illustrates the role of rate of wetting as a means ofcontrolling slaking. Le Bissonnais and Arrouays (1997) also examined the relationship between aggregatestability (as measured by soil crusting and erodibility) and different types and amounts of organic matter.

RELATIONSHIP OF LABORATORY AND FIELD METHODS OF MEASURING SOIL ERODIBILITY

The previous sections all discuss various methods used to measure soil structure breakdown on wetting in thelaboratory, with particular emphasis on the physico-chemical factors controlling clay dispersion and surface



Figure 9. Effect of organic matter in preventing incipient failure (adapted from Quirk and Panabokke, 1962, by permission of BlackwellPublishing)

crusting (Rengasamy et al., 1984). The physico-chemical factors controlling clay dispersion have also beenshown to be important in other laboratory experiments investigating the effects of water chemistry on soilhydraulic properties under simulated rainfall. For example, in the work by Borselli et al. (2001), using simulatedrainfall, it was shown that soil structure decline and erosion occurred at a greater rate when demineralized waterwas used when compared to tap water (which contained 30–50 µS cm−1 of salts).

However, there is a general lack of calibration of these laboratory measures of soil structural breakdownagainst field parameters of interest, such as infiltration and erodibility. Agassi and Bradford (1999) also pointout the problems related to both laboratory and field experiments using rainfall simulators because in bothcases it is difficult to replicate characteristics of specific natural rainfall events. Several investigators havefound that aggregate stability measurements carried out in the laboratory are usually inconsistent and lackcorrelation with field soil behaviour (Loveday, 1980; Moldenhauer, 1980; Loch 1994). Thus we need methodsthat can be related to the field behaviour of soils. These methods need to take account of the properties of thesoil in terms of chemistry, physics and erosion mechanics. The following sections discuss the relationshipsbetween laboratory and field measures of erosion behaviour, from the viewpoints of both detachment andsediment concentration.

Aggregate stability and soil detachment

Loch and Foley (1988) and Coughlan et al. (1991) discussed a novel method for relating laboratory and fieldbehaviour, which involves measuring aggregate stability after subjecting a field soil to rainfall wetting. Simu-lated rainfall wetting is used and aggregate breakdown in the surface seal layer is measured using wet sieving.Figure 10 shows that for a range of Queensland soils there is a significant (R2 = 0·871) relationship between thefinal infiltration rate (If ) and the percentage of aggregates <0·125 mm in size. The size range <0·125 mm issignificant in that it is particularly detrimental to soil structure, causing If to decrease, and a crust/seal to format the soil surface. Even more important though from an erosion point of view, the decrease in If with increasedslaking into material <0·125 mm means that there will be more runoff and therefore more water will be availablefor transport processes. This increase in runoff, combined with more finer materials already detached, results ingreater potential for erosion.

Geeves et al. (1995) used rainfall–runoff simulations on re-packed soils to quantify the effects of aggregatebreakdown and seal formation on infiltration rates. They also found that the percentage of particles <0·125 mm



diameter predicted the decline in infiltration rates. Rate of wetting effects on aggregate breakdown were alsodouble the magnitude of effects due to raindrop kinetic energy levels. Fox and Le Bissonnais (1998) were alsoable to correlate aggregate stability indices with soil erosion rates. Sieved soil aggregates were packed into traysand rained on for four hours at 23 mm h−1. They showed that the higher the stability of the soil (measured usingmean weight diameter (MWD)) the lower was the soil loss rate since breakdown products were coarser and lesstransportable.

Besides rainfall simulation experiments, investigations of crust typology have been used to relate laboratorymeasurements to processes of runoff and erosion in the field. Bresson and Valentin (2001) suggested a morpho-genetic classification of crusts based on morphology characterization that helped to assess crusting rate as wellas the processes involved. This approach gave insight into predicting crust formation depending on soil materialcharacteristics, climatic conditions and management practices, as well as selecting the most suitable controltechniques.

Aggregate breakdown and sediment concentration

The above examples illustrate that considerable progress has already been made in understanding the elemen-tary processes involved in aggregate breakdown and soil detachment, and in relating laboratory measurementsof aggregate stability and crust characteristics to erosion processes in the field. However, the rate of soil lossfrom a hillslope is controlled by a combination of the rate of soil detachment and the ability of the overland flowto carry this sediment. So far in the discussion of the consequences of elemental processes we have focused onthe first part of this combination. Here we briefly examine the direct influence of the detachment of sedimentof varying particle sizes upon the sediment transport process.

Some sediment transport equations (e.g. Yang, 1973) relate sediment transport capacity to the settling velocityof the sediment being carried. Other relationships assume sediment transport capacity is independent of the sizeor settling velocity of the sediment (e.g. Bagnold, 1966). If the former approach is used, dispersion or otherforms of aggregate breakdown, which occur during the detachment processes, result in higher, predicted sedi-ment transport capacities. Conversely, maintaining stable aggregates reduces the capacity of overland flow tocarry sediment from a soil surface.

The models developed by Hairsine and Rose (1991, 1992) permit the consideration of this effect for soileroded across a wide range of sediment size classes. In these models sediment concentration is predicted to beinversely proportional to the depositability, (1/I)Σ I

i=1 vi, where I is the number of settling velocity classes eachwith equal mass of detached sediment and vi is the settling velocity of class i. The depositability is dominatedby the proportion of sediment with larger values of vi. Thus the breakdown of larger sediment into smallersediment results in significant predicted change in predicted sediment transport.

Figure 10. Effect of aggregate breakdown into particle <0·125 mm diameter on final infiltration rate (adapted from Loch and Foley, 1994,by permission of CSIRO Publishing)



CONCLUSIONS

The interaction of water with clay surfaces and its effects on soil swelling, dispersion and slaking are key processesinvolved in soil detachment and subsequent erosion. These interactions and their effects on soil stability needto be considered in relation to the threshold electrolyte concentration required to keep a soil flocculated. Usingthe SAR/EC diagram and the conditions for clay dispersion/flocculation under mechanical conditions, soils canbe shown to exist in one of three regions, each representing different management requirements. The threeregions are dispersed, flocculated, or variable resilience. Minimum disturbance and/or addition of electrolytesare required in the dispersed region, while in the flocculated region maintenance of organic matter and/or normalsoil conservation practices are required. In the region of variable resilience, soil behaviour can be controlledeither in the short term by increasing cover levels, or in the long term by increasing soil organic matter.

Even though several of the linkages between surface soil properties, the factors controlling aggregate break-down, the role of cover types, and subsequent sediment transport are not fully understood and require furtherinvestigation, there are significant well-established conclusions from the literature that can be stated. Firstly,cover of the soil surface either by vegetative material or other forms of litter or cryptogams, is a major controlmeasure for reduction of rainfall-driven and overland flow-driven erosion. Secondly, it is likely that the break-down of aggregates into finer particles during detachment is likely to have an impact on the rates of soil erosion,though not all predictive models describe this. Finally, cover fulfils a wide range of roles in protecting soilsurface from structural breakdown and erosion. These roles include direct interception of rainfall energy, reduc-tion of the velocity of the overland flow, and the enhancement of the depth of surface water that in turn reducesraindrop impact on the soil surface.

ACKNOWLEDGEMENTS

Many colleagues have contributed to this paper through informal discussions. The authors are particularlygrateful to Clive Hilliker for his assistance with the diagrams, and Dr Guy Geeves for comments on an earlierdraft. Comments by two anonymous referees were also appreciated. Jeremy James is thanked for the photo ofthe gully.

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